The subject matter disclosed herein generally relates to methods and apparatus for determining concentrations of gas mixtures.
With modern sophisticated chemical analytical techniques such as gas chromatography—mass spectrometry, it is generally possible to determine the composition of gas mixtures with a relatively high degree of precision. However, such sophisticated techniques can be time consuming and expensive, and simpler, faster, and/or less expensive techniques are often desired. In some situations, the identity of the components in a gas mixture may already be known with reasonable certainty, and it is only necessary to determine the concentrations of the already-identified components of the mixture.
One example of this is with a refrigeration system such as a chiller or air conditioning system having a heat transfer loop with a refrigerant flowing through it. In the past, refrigerants used in heat transfer loops often consisted of a single compound such as R-12, R-22, or R-134a. However, increasing demands for refrigerants that can meet demanding specifications across a number of parameters, such as heat transfer performance, ozone depletion potential (ODP), global warming potential (GWP), toxicity, and/or flammability, have necessitated that blends of different compounds for use as refrigerants in order to provide desired performance. The use of refrigerant blends, however, can lead to problems in monitoring and maintaining refrigeration system performance. For example, refrigeration systems can be prone to developing leaks in the refrigerant loop. With a single-compound refrigerant, a small leak may not have a significant adverse impact on system performance until a substantial quantity of refrigerant has leaked out of the system. With a blended refrigerant, however, a leak can cause fractionation, which alters the composition of the refrigerant blend remaining in the system and can adversely impact refrigerant properties or performance. Therefore, it is desirable to be able to determine the concentration of components in a refrigerant blend.
Attempts have been made to determine refrigerant blend compositions by monitoring the refrigerant state (e.g., temperature and pressure) at different locations in the refrigerant loop. For example, U.S. Pat. No. 6,079,217 discloses a refrigeration system that attempts to determine the composition of a ternary blend of four refrigerants by measuring the refrigerant states (e.g., pressure and temperature) at the inlet and outlet of expansion device, such that the composition of blend can be determined based on the isenthalpic assumption and the vapor-liquid-equilibrium diagram. Such attempts, however, are subject to a number of disadvantages, such as only being useful for non-azeotropic blends, lack of portability, and they require the permanent installation of costly temperature and pressure sensors and control subsystems in each refrigeration system.